API Architect

api-architect — design the contract before writing the handler

When to use this skill

Trigger when the user needs an API design before implementation. Strong signals:

• "design an API for "
• "what should the endpoints look like?"
• "REST or GraphQL for this use case?"
• "give me an OpenAPI spec for X"
• A feature spec or PRD pasted with no endpoint plan

Do not trigger for: small additions to an existing API (just add the endpoint, matching local conventions), pure data modeling (use schema-architect), or when wrapping an existing API (use mcp-forge).

The output contract

A design artifact that includes:

1. A short rationale — 1 page. Why REST vs GraphQL, what trade-offs were made, what was deliberately left out.
2. The schema — OpenAPI 3.1 YAML for REST, or a GraphQL SDL with typed resolvers planned.
3. Resource model — what the nouns are, what the verbs are, what the relationships are.
4. The boring-but-critical parts — auth, pagination, filtering, errors, versioning. All explicit, all consistent.
5. A "what's NOT in v1" section — so reviewers don't argue about features that were intentionally deferred.

Workflow

1 — Read the requirement, find the resources

From the spec, list:

• Nouns: the things users will create, read, update, delete (Order, Customer, Invitation)
• Verbs: the actions that don't fit CRUD (/orders/{id}/cancel, /invitations/{token}/accept)
• Queries: how users will find lists (my open orders, customers signed up this week)
• Side effects: who needs to be notified, what gets emailed, what gets logged

This list is the design surface. Everything below it is a choice you make about that surface.

2 — REST or GraphQL?

Don't default. Choose deliberately:

Choose REST when:

• Consumers are diverse (browser, mobile, third-party integrations)
• The data shape is mostly resource-oriented and predictable
• You want HTTP caching, CDNs, easy debugging in the browser network tab
• Operations are uniform CRUD on clear resources

Choose GraphQL when:

• One client (typically a complex SPA) fetches deeply nested, varied shapes
• Over-fetching is a real perf problem and you've measured it
• The team can absorb the operational complexity: resolver perf, N+1 protection, query depth limits, persisted queries

Write the choice + 2 reasons in the rationale. If you can't articulate it, default to REST.

3 — Design resources

For REST:

• URLs are nouns, plural, kebab-case: /customers, /api/v1/invoice-line-items
• HTTP verbs do the work: GET /orders, POST /orders, GET /orders/{id}, PATCH /orders/{id}, DELETE /orders/{id}
• Sub-resources for clear ownership: GET /orders/{id}/line-items (when line items have no independent existence)
• Actions that don't fit CRUD become POSTs to a sub-route: POST /orders/{id}/cancel, POST /invitations/{token}/accept
• IDs are stable, opaque, never sequential integers in public APIs (use ULIDs, UUIDs, or prefixed IDs like cus_abc123)

For GraphQL:

• One Query root for reads, one Mutation root for writes
• Nodes implement a Node interface with a global ID
• Connections for pagination follow Relay spec (edges, node, pageInfo)
• Mutations take a single input object: signUp(input: SignUpInput!): SignUpPayload!

4 — Auth model

Decide once. Stick to it.

• Bearer tokens (JWT or opaque): Authorization: Bearer <token> on every request
• API keys: header (X-API-Key) not query string
• OAuth 2.0: spec the scopes per endpoint
• Cookie sessions: only for first-party browser clients

Document:

• Which endpoints are public
• Which require auth and what scopes/roles
• What 401 vs 403 means in this API

5 — The boring-but-critical layer

Specify these once, apply everywhere:

Pagination (cursor, not offset, for anything that might grow):

GET /orders?cursor=<opaque>&limit=50
→ { data: [...], next_cursor: '...', has_more: true }

Filtering: explicit query params (?status=open&customer_id=cus_123), not a generic filter= blob.

Sorting: ?sort=created_at or ?sort=-created_at (leading - for desc).

Errors: pick a format and use it everywhere. RFC 7807 (application/problem+json) is the safest default:

{ "type": "/errors/insufficient-funds", "title": "Insufficient funds", "status": 402, "detail": "Account balance is $4.50, charge was $10.00", "instance": "/orders/ord_123" }

Versioning: URL path for major (/api/v1/, /api/v2/). Deprecation headers for warnings. Never minor-version a URL.

Idempotency for unsafe operations: accept Idempotency-Key: <uuid> header on POSTs that create resources or move money.

Rate limits: return X-RateLimit-Limit, X-RateLimit-Remaining, X-RateLimit-Reset headers and 429 with Retry-After.

6 — Produce the spec

For REST: a complete OpenAPI 3.1 YAML, validated with redocly lint or swagger-cli validate. Every endpoint has request schema, response schemas (including the error envelope), and example.

For GraphQL: a complete SDL file. Every type, every field, every input. Use @deprecated(reason: ...) rather than removing fields.

7 — Write the rationale

One page, plain prose. Cover:

• Why REST or GraphQL
• The auth choice + reasoning
• The pagination/error choices + reasoning
• What's explicitly out of scope for v1
• The biggest trade-off you made and what would change the call

Patterns and anti-patterns

✅ Do:

• Make 201 responses return the created resource (or at least its ID + canonical URL).
• Use 422 for validation errors, 400 only for malformed requests.
• Make DELETE idempotent — second call returns 204 or 404, not 500.
• Return ISO 8601 timestamps in UTC, always. Never Unix timestamps in public APIs.
• Treat the spec as the source of truth; generate clients and types from it.

❌ Don't:

• Don't expose database column names as field names. The DB schema is yours to change; the API is a contract.
• Don't paginate with ?page=N&pageSize=M for anything write-heavy — race conditions skip items.
• Don't reuse HTTP status codes ambiguously. 404 means "no resource"; don't also use it for "you don't have permission to see this resource" (that's 403, possibly disguised as 404 for security).
• Don't put auth tokens in URLs. Logs, browser history, referrers all leak them.
• Don't add a success: true envelope. HTTP status is the envelope.

Example invocation

User: "Design the API for an invitation system. Inviter creates invitations; invitee accepts via email link."

1. Read spec, list resources: Invitation (id, inviter_id, email, role, token, status, expires_at).
2. Verbs: create invitation, list my invitations, revoke invitation, accept invitation (one-time, by token).
3. Choose REST (multiple clients: web + email links + integration partners).
4. Rationale: REST chosen for HTTP-caching the public accept page and simple email link semantics. Auth: bearer JWT for inviter routes, public + token for accept.
5. Endpoints:
   • POST /api/v1/invitations (auth required) → 201 + invitation
   • GET /api/v1/invitations?status=pending (auth required) → paginated list
   • DELETE /api/v1/invitations/{id} (auth required) → 204
   • GET /api/v1/invitations/by-token/{token} (public, rate-limited) → invitation preview
   • POST /api/v1/invitations/by-token/{token}/accept (public, requires registered user) → 200 + membership
6. Errors: RFC 7807 envelope. Pagination: cursor-based. Idempotency: Idempotency-Key on POST /invitations.
7. Out of v1: bulk invite, custom roles per invitation, branded email customization.
8. Output: OpenAPI YAML + 1-page rationale in docs/api-invitations.md.

See also

• schema-architect — translates the resource model into the DB schema
• mcp-forge — wraps the finished API as a Claude-callable surface
• doc-craft — turns the spec into developer-facing API docs